Loss of muscle tissue often occurs as a result of aging, malnutrition, and catabolic diseases such as burns, sepsis, and cancer. Dietary protein supplementation may be beneficial, but supplementation with the essential amino acid leucine has been shown to be especially beneficial. Dietary leucine has, for example, recently been shown to suppress the rate of myofibrillar protein degradation and muscle weight loss in rats. Leucine also stimulates muscle protein synthesis and modulates the activity of various proteins involved in the control of mRNA translation. Leucine may stimulate protein synthesis directly or through its metabolite, α-ketoisocaproic acid. Leucine may stimulate translation either independently or by interaction with the mammalian target of rapamycin (mTOR).
Leucine is one of the branched-chain amino acids and is an essential amino acid. It is the only amino acid that is converted to acetyl-coenzyme A and alpha-ketoacids and is an important source of nitrogen for synthesis of glutamine. In addition to its effects on protein synthesis and degradation, leucine also stimulates glucose uptake by protein kinase C (PKC), while insulin modulates glucose uptake via protein kinase B.
Milk-derived whey is one good source of muscle-building proteins. Whey protein isolates and whey protein concentrates are used in protein-building dietary supplements. Whey proteins are a good source of leucine, but for many individuals who need the muscle-building amino acids dietary proteins can provide, it is difficult to digest and/or absorb proteins. It is therefore important to find ways to provide leucine-containing amino acid compositions that provide amino acids in a more bioavailable form for improving muscle protein synthesis.